The invention relates to a carrier assembly for shaped charges for use in a perforating gun of the type generally used to perforate oil and gas wells and method of assembling same. More particularly, the invention relates to an improved retaining clip for holding a detonating cord adjacent a rear end of a shaped charge and for securing the shaped charge in place within a carrier assembly in a perforating gun.
In a common embodiment, perforating guns may be comprised of a closed, fluid-tight reusable or expendable hollow carrier gun body adapted to be lowered on a wire line or tubing conveyed into a well bore. In an expendable type hollow carrier gun, a tubular charge holder (hereinafter referred to as the xe2x80x9ctubular charge holderxe2x80x9d), is disposed within the hollow carrier gun body. A plurality of shaped explosive charges (hereinafter xe2x80x9cshaped chargesxe2x80x9d) are disposed in the tubular charge holder in a selected spatial distribution. A detonating means, such as a detonating cord, is positioned adjacent to the rear of the shaped charges. The shaped charges have conically constrained explosive material therein. The shaped charges are typically distributed over the length of the gun. Detonation of the charges can be activated electronically or mechanically depending upon the technique used for lowering the perforating gun in the well bore. The detonating means is activated from the surface when the perforating gun has been positioned in the well bore adjacent to a subterranean formation to be perforated.
In a closed, fluid-tight type of gun body, the explosive jets produced upon detonation of the shaped charges penetrate the hollow carrier gun body before penetrating the casing wall of the well bore and the adjacent formation. To reduce the resistance produced by the hollow carrier gun body and increase the depth of perforation penetration into the formation, the perforating gun body may be provided with external recesses or scallops that leave relatively thin wall portions through which the explosive jets pass. The scallops in the hollow carrier gun body must be positioned in a spatial distribution that corresponds to the spatial distribution of the shaped charges held within the gun body by the tubular charge holder.
In an expendable hollow carrier gun, the tubular charge holder may be formed from thin walled tubes of metal, cardboard, plastic or other materials having openings cut into the tube wall for receiving the shaped charge.
Frequently, but in not all cases, it is desirable to produce a shot pattern that emanates in various directions from the gun. This is known in the art as shot phasing. Phasing may range from 0E (all shots in alignment) to any choice of shot pattern. Typical shot phasing may be spaced at 45E, 60E, 90E or 120E. To produce a shot pattern emanating in various directions, the charges are usually circumferentially offset, i.e., the discharge end of each successive shaped charge is laterally spaced from a preceding charge whereby the discharge ends of the charges as well as the corresponding external recesses or scallops in the hollow carrier gun body lie on a helical path. The successive detonation of the charges along the helical path produces a shot pattern and perforations around and over the length of the gun.
In an expendable hollow carrier type gun, the shaped charges must be secured in the tubular charge holder and the detonating cord used to detonate the shaped charges must be retained in contact with the center of the rear surface of each shaped charge to assure that the shaped charge is detonated. Retaining clip type structures have been utilized with shaped charges to hold a detonating cord and/or to hold the shaped charge in place relative to the tubular charge holder. For example, U.S. Pat. No. 4,312,273 to Camp discloses a shaped charge mounting system including a spring wire clip 30 which holds the shaped charge in place and which includes arms which aid in holding a detonating cord. U.S. Pat. No. 3,444,810 to Hakala discloses a shaped charge assembly including a swivel clamp base 38 which has a pair of resilient prongs 40 for holding a detonating cord. U.S. Pat. Nos. 4,479,556 to Stout et al., 4,326,462 to Garcia et al. and 2,756,677 to McCullough disclose shaped charge holder assemblies including various types of attachment means for holding the shaped charge in place a tubular charge holder. U.S. Pat. Nos. 4,583,602 to Ayers and 2,764,938 to Harcus both disclose tubular type charge holder assemblies in which shaped charges are held within pairs of diametrically opposed circular openings in the tubular charge carrier. U.S. Pat. No. 4,832,134 to Gill discloses a retaining clip operably associated with a flange on a rearward end of a shaped charge for retaining the shaped charge within a tubular type charge holder in which shaped charges are held within pairs of diametrically opposed circular openings in the tubular charge carrier.
In commonly used prior art reusable ported type hollow carrier gun bodies, the basic shaped charges are typically not held in a tubular charge carrier. The shaped charges typically have a flat rear surface and are inserted from one end of the hollow carrier with a special tool to a location proximate a ported opening. The front face of the shaped charge is directed out of the ported opening. A detonating cord runs inside of the hollow carrier gun body for detonating the shaped charges. The shaped charge typically includes a polymeric sleeve surrounding the exterior of the charge used for holding the detonating cord proximal to the rear of the charge. An alignment washer (face cover) is disposed over the front of the charge. A threaded plug is screwed into the ported opening and contacts the face cover of the charge to secure the shaped charge in place within the hollow carrier gun body.
A need exists for a reliable and economical tubular charge holder assembly for use with an expendable type hollow carrier perforating gun that further includes a less expensive basic shaped charge. The present invention provides a reliable clip for both securing a less expensive basic shaped charge in a tubular charge holder used internally in a hollow carrier gun and concurrently securing the detonation cord in proximity to the center point of the rear surface of the shaped charge.
A tubular charge holder assembly made of an expendable tube having a plurality of forward openings and a plurality of corresponding rear openings that are diametrically opposed from the forward openings. The openings are formed over a length of the tube, wherein the forward openings are larger than the rearward openings. In one embodiment, the forward and rearward openings may be angularly offset from corresponding forward and rearward openings immediately above and below. In an alternate embodiment having OE shot phasing, the forward openings may be in alignment with one another and the rearward openings in alignment with one another. Shaped charges are located within the openings in tubular charge holder. The shaped charges have a case having a rim surrounding a front face and having a rearward end having a rear face with a center point. The shaped charge contains a shaped explosive and a cone within the case. The shaped charge is held within the tubular charge holder so that a portion of a rim on the front face is accessible proximate the forward opening of the tubular charge holder and at least a portion of the rear face is accessible proximate the rearward opening of the tubular charge holder.
A detonating cord is wrapped around the expendable tube of the tubular charge holder such that the detonating cord traverses the length of the tube and helically wraps around the tube. The detonating cord is positioned so that it is proximate to the rear surface of each of the shaped charges. The detonating cord may be wrapped around the outside of the tube. Alternatively, the detonating cord may pass on the inside of the tube when traversing from charge to charge and pass outside the tube when disposed across the rear face of the charge. A substantially semi-circular clip is positioned on the outside of the tube partially surrounding the tube. The clips have a first hook on a first end and a second hook on a second end on the front of the clip. On the back of the clip, the clip has a retainer, such as a U-shaped portion or snap device to secure the detonating cord. The retainer may be perpendicular to the clip or may be angled or offset with respect to the clip to accommodate the helically wrapped detonating cord. The clip may be made of a wire, a pair of wires, a band, a perforated band having a adjuster or of other suitable configurations.